1. Field of the Invention
This invention relates generally to combustion or gas turbines, and more particularly to the compressor diaphragm assemblies that are typically used in such turbines.
2. Statement of the Prior Art
Over two-thirds of large, industrial combustion turbines (which are also sometimes referred to as "gas turbines") are in electric-generating use. Since they are well suited for automation and remote control, combustion turbines are primarily used by electric utility companies for peak-load duty. Where additional capacity is needed quickly, where refined fuel is available at low cost, or where the turbine exhaust energy can be utilized, however, combustion turbines are also used for base-load electric generation.
In the electric-generating environment, a typical combustion turbine is comprised generally of four basic portions: (1) an inlet portion; (2) a compressor portion; (3) a combustor portion; and (4) an exhaust portion. Air entering the combustion turbine at its inlet portion is compressed adiabatically in the compressor portion, and is mixed with a fuel and heated at a constant pressure in the combustor portion, thereafter being discharged through the exhaust portion with a resulting adiabatic expansion of the gases completing the basic combustion turbine cycle which is generally referred to as the Brayton, or Joule, cycle.
As is well known, the net output of a conventional combustion turbine is the difference between the power it produces and the power absorbed by the compressor portion. Typically, about two-thirds of combustion turbine power is used to drive its compressor portion. Overall performance of the combustion turbine is, thus, very sensitive to the efficiency of its compressor portion. In order to ensure that a highly efficient, high pressure ratio is maintained, most compressor portions are of an axial flow configuration having a rotor with a plurality of rotating blades, axially disposed along a shaft, interspersed with a plurality of inner-shrouded stationary vanes providing a diaphragm assembly with stepped labyrinth interstage seals.
A significant problem of fatigue cracking in the airfoil portion of inner-shrouded vanes exists, however, due to conventionally used methods of manufacturing such vanes. For example, in either of the rolled or forged methods used by the manufacturers of most compressor diaphragm assemblies, a welding process is used to join the vane airfoils to their respective inner and outer shrouds, such process resulting in a "heat-affected zone" at each weld joint. Crack initiation due to fatigue, it has been found, more often than not occurs at such heat-affected zones. Therefore, it would be desirable not only to provide an improved compressor diaphragm assembly that would be resistant to fatigue cracking, but also to provide a method of fabricating such assemblies that would minimize processes which produce heat-affected zones.
The problems associated with fatigue cracking are not, however, resolved merely by eliminating those manufacturing processes that produce heat-affected zones. That is, it is well known that certain forged-manufactured vane airfoils, even after having been subjected to careful stress relief which reduces the effects of their heat-affected zones, can experience a fatigue cracking problem. It is, therefore, readily apparent that not only static, but also dynamic stimuli within the combustion turbine contribute to the problem of fatigue cracking.
Forces that act upon the inner shroud and seal of a compressor diaphragm assembly are due, primarily, to seal pressure drop. Those forces, as well as aerodynamic forces acting normally and tangentially upon, and distributed over the surfaces of the vane airfoil, each contribute to the generation of other forces and moments that are transferred to the outer shroud, and subsequently to the casing of the combustion turbine via the weld joints which attach the vane airfoil to the outer shroud.
It would appear that the simple alternative of using vane airfoils with integral outer and inner shrouds would quickly solve both causes of fatigue cracking. That is, the problem of heat-affected zones would appear to be eliminated entirely while the problems associated with instabilities due to static and dynamic stimuli within the combustion turbine would appear to be minimized. Such is not the case, however.
For example, under the influence of the static forces and moments described above, the outer shroud segment of this hypothetical vane airfoil would not be stably engaged within the casing of the combustion turbine until such time that a restraining moment could be generated by contact of the extremities of the outer shroud segment with the walls of the slot formed in the casing to receive the segment. The outer shroud segment would, thus, rotate within the clearance gap (provided in the casing slot to account for thermal expansion). As a result, use of the hypothetical vane airfoil in a combustion turbine would lead to a great deal of stress in the vicinity of the outer shroud segment and excessive translational and rotational displacements, each of which would be further exacerbated under dynamic stimuli. It would also be desirable, therefore, to provide an improved compressor diaphragm assembly that would avoid the above described instabilities of engagement.